Chemistry
Osmolar Gap
Measured minus calculated osmolality.
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Background
The osmolar gap is the difference between the directly measured serum osmolality and a calculated estimate, used to detect unmeasured, osmotically active substances such as the toxic alcohols methanol and ethylene glycol. The calculated osmolality is derived from the routine osmoles — sodium, glucose and urea — using the formula 2×Na + glucose + urea (all in mmol/L). A large gap suggests that additional osmoles are present that the calculation does not account for. It is reported in mOsm/kg.
Interpreting the result
A gap below about 10 mOsm/kg is normal, while a value of 10 or above is raised and should prompt consideration of toxic alcohols or other unmeasured osmoles (for example mannitol, or alcohol from a recent drink). An elevated gap is supportive but not diagnostic, and a normal gap does not exclude ingestion, particularly late in the course when the parent alcohol has been metabolised. It is interpreted together with the clinical picture, the anion gap and an acid–base assessment.
Worked example
A patient has a measured osmolality of 320 mOsm/kg, sodium 140, glucose 6 and urea 5 mmol/L. The calculated osmolality is (2×140) + 6 + 5 = 291, so the osmolar gap is 320 − 291 = 29 mOsm/kg — markedly raised, prompting consideration of a toxic-alcohol ingestion.
Critical actions
Calculated osmolality = 2×Na + glucose + urea (mmol/L). A normal gap does not fully exclude early toxic-alcohol ingestion. Discuss with toxicology if suspected.
Pearls / pitfalls
- A normal osmolar gap does not exclude toxic-alcohol poisoning — late in the course the alcohol is converted to acids and the anion gap rises instead.
- Baseline gaps vary between individuals, so a "normal" gap may still represent a rise from that person's own baseline.
- Ethanol can raise the gap and should be accounted for; many laboratories add an ethanol term when it is known.
- Interpret with the anion gap and acid–base status, and discuss with toxicology when ingestion is suspected.
Evidence & validation
The concept is long established in clinical toxicology and laboratory medicine; the normal upper limit is conventionally taken as about 10 mOsm/kg, though there is wide individual variation in the baseline. It is used in poisoning guidance as one strand of evidence when toxic-alcohol ingestion is suspected.
Frequently asked questions
What is a normal osmolar gap?
Conventionally a gap below about 10 mOsm/kg is considered normal. There is wide variation between individuals, so this threshold is a guide rather than an absolute rule.
Does a normal gap rule out methanol or ethylene glycol poisoning?
No. Early on the gap may be raised, but as the alcohol is metabolised the osmolar gap falls and the anion-gap acidosis develops. A normal gap, especially later, does not exclude ingestion.
How is the calculated osmolality worked out?
A common formula is 2×sodium + glucose + urea, with all values in mmol/L. The measured osmolality minus this calculated value gives the osmolar gap.
Why should I look at the anion gap too?
Toxic alcohols cause a raised osmolar gap early and a high anion-gap metabolic acidosis later as they are metabolised. Using both, alongside the clinical picture, improves detection across the time course of ingestion.
References
- Hoffman RS, Smilkstein MJ, Howland MA, Goldfrank LR. Osmol gaps revisited: normal values and limitations. J Toxicol Clin Toxicol. 1993;31(1):81–93.
- Kraut JA, Mullins ME. Toxic Alcohols. N Engl J Med. 2018;378:270–280.
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